Just installed barends for the first time in five years and hey: they wouldn't clamp. Of course they were gucci $30 things that weight too little, and thus have a poor clamping surface 'cause they're machined out.
I bought a pair of $8 barends and they work fine. Moral of the story: check the clamping surface first. Is it big enough? If so, grease the bolt. Sometimes you just need a tad more torque on the bolt, and a dry or rusty thread doesn't help.